1. Field of the Invention
This invention relates to a method of processing auscultation signals, consisting of the steps of receiving an auscultation signal and providing a biased auscultation signal, and calculating a signal representative of an estimated rhythm of the auscultation signal.
Moreover, the present invention relates to an apparatus for processing auscultation signals, and in particular to an electronic stethoscope suitable for use in cardiology.
2. Description of Related Art
In recent years physicians have had an impressive arsenal of instrumentation at their disposal for the diagnosis of cardiovascular diseases. One such instrument is the well known stethoscope used to detect sounds originating from the heart and adjacent large vessels. With the introduction of the stethoscope, auscultation grew into an art providing the clinician with valuable information of the functional integrity of the heart. Nowadays it is possible to process the information residing in the auscultation signal electronically by use of embedded knowledge obtained by clinical research. Therefore it is possible to relieve the clinician from trivial tasks. One such task may be to estimate the heart rate of a beating heart based on the auscultation signal.
The auscultation signal consists of characteristic sounds that originate from the contraction and expansion of the heart in the process of pumping blood, i.e. sounds caused by vibrations from acceleration and deceleration of blood. When a beating heart is auscultated it is not the beat frequency that is heard, but rather the characteristic sounds. Further, for mid-aged healthy persons in normal conditions, the characteristic sounds often denoted S1 and S2 related to a contraction and expansion of the heart, respectively, are separated in time by a sufficiently large interval that allows for perception of the characteristic sounds as individual sounds, i.e. the characteristic sounds, are typically separated by more than 150–200 milliseconds for adults and about 50 milliseconds for children. Typically, the characteristic sounds have a continuous frequency spectrum distributed between approximately 15 Hz and 100 Hz and peaking at 30–40 Hz.
However, the characteristic sounds may be superimposed with sounds originating from different types of vibrations in the heart. These sounds are the so-called murmurs, that may be caused by mechanisms such as blood turbulence or flutter of structures. Murmurs may be divided into two primary groups: a first group consists of random ‘noisy’ sounds and a second group consists of ‘musical’ sounds with a continuous narrow-band frequency spectrum. Experience has shown that in a human being the ‘musical’ sounds are typically related to the heart beat frequency. The frequency spectrum of murmurs may be distributed up to 500–2000 Hz.
The characteristic sounds may also be disturbed by spurious contractions of the heart appearing at certain or random points in time. Further, the characteristic sounds may also appear at random points in time. This phenomenon is known as arrhythmia.
When the sound of a beating heart including murmurs, if any, is auscultated by means of a transducer placed on e.g. the chest of a patient, then low-frequent rumble noise will be picked up by the transducer. This rumble noise may have a frequency content primarily distributed from just below the frequency of the heart rate towards DC.
In severe cases these disturbances are able to destroy the characteristic sounds in the auscultation signal completely or at least disturb the auscultation signal to such a degree that it may be impossible to estimate even an approximated heart rate by use of the prior art technology.
However, for a trained person using a non-electronic stethoscope, the information in the form of characteristic heart sounds is relatively easily recognisable from repetition to repetition and therefore relatively easy to count. This is due to the fact that the human perception of the auscultation signal is very good at diminishing the disturbances or artefacts and pay attention to the features in the auscultation signal that provides information for estimating the heart rate.
U.S. Pat. No. 4,972,841 discloses an electronic stethoscope with a pulse rate display for displaying an estimated pulse rate. In the stethoscope a transducer converts so-called Korotkoff sounds into electrical signals. Korotkoff sounds are sounds created by vibrating walls of collapsed vessels as blood is just allowed to flow through the vessel. The electrical signals are amplified and fed to a counter in which the detected pulse rate per unit time is calculated and the result is displayed as a digital pulse rate.
However, this patent is not concerned with estimation of heart rates originating from humans where the measured sound may be very different from expected normal heart sounds. The patent does not disclose any means adapted to suppress artefacts that may be confused with or mixed up with the information in the auscultation signal relating to the heart rate.
U.S. Pat. No. 4,436,096 discloses a digital heart rate stethoscope. A transducer produces electrical signals representing the heart sound of a human patient and a filter is connected to receive the electrical signals. The filter is a narrow-band band-pass filter whose passband is centered on a characteristic heart sound frequency e.g. 33 Hz. The filter is thereby adapted to improve the signal-to-noise ratio and enables the transducer to be used over a patient's clothing. The filtered signal is converted into pulses which are counted in a counter and displayed as a digital heart rate indication.
Again, this patent is not concerned with estimation of heart rates originating from humans where the measured sound may be very different from the expected normal heart sounds. Neither does this patent disclose any means adapted to suppress artefacts that may be confused with or mixed up with the information in the auscultation signal relating to the heart rate.
International Application WO 97/00045 discloses a stethoscope for examining the characteristic of sound waves made by a beating heart. The stethoscope comprises means for establishing a threshold wave form beyond which cyclic reference complexes appear and are identified, and a time measurement device for measuring the time elapsed between similar moments on adjacent reference complexes as an indicator of rate of heart beat.
In this stethoscope the heart rate is estimated based on the assumption that there is substantially no noise between the heart sounds originating from contraction and expansion of the heart. The stethoscope is therefore not suited for use of estimating heart rates of persons suffering from heart murmur or arrhythmia.
Other apparatuses for estimating the heart rate use means for determining the envelope of the auscultation signal. The envelope is determined by taking the absolute value of the auscultation signal and band-pass filtering the absolute value signal.